Longitudinal analysis of sleep in relation to BMI and body fat in children: the FLAME study

Carter et al1 in their analysis of sleep in relation to BMI and body
fat in children, concluded that insufficient sleep leads to becoming
overweight. A 1h difference in night sleep, between 3y and 5y, was
associated with a significant BMI difference of 0.56 at age 7y (about
0.48kg fat mass). However, such a weight gain in shorter sleepers must be
balanced against the probability that it accumulates over years of
sleeping like this2; maybe amounting to hundreds of hours of 'less
sleep'.

The relatively small impact of short sleep on obesity is reflected by
another study3 on sleep and BMI in 3857 infants and 3844 children,
spanning ages 0-7y. Sleep duration did not predict obesity at any age
except at 6-7y, when obese children slept 30min less. These authors3
viewed sleep interventions as 'premature'. There is no evidence that
30min or even 1h extra sleep in an overweight child will obviate weight
gains2. Even early 'prophylactic' sleep interventions with poor sleeping
infants will not reduce their becoming overweight4.

By looking at this type of data in terms of those who are not obese,
the outcome is, ostensibly, of lesser concern. For example, another
study5 (in the BMJ) identified eight early life risk factors for obesity,
including short sleep (odds ratio 1.45). However, if these findings5 are
'inverted', 89.7% of short (<10.5h) sleepers were not obese, compared
with 93.4% 'non-obesity' for longer (11-12h) sleepers; only a 3.7%
difference.

Apart from better dietary management, then rather than extend sleep
in an attempt to normalise weight in obese children, a small portion of
this extra sleep spent in physical exercise is far more likely to achieve
such ends.

A related issue, noted by Carter et al1, are claims that today's
children are sleeping less, which might further compound this obesity
situation. However, the recent, substantial review6 of the 51 studies of
secular trends in children's sleep found, at best, only a "limited
scientific basis"6 for any such sleep reduction. Interestingly, an often
cited substantial study7 from 1913, reported an average 11.2h sleep
duration for 6-8y year olds, which seems identical to that reported by
Carter et al.

Similar overall observations apply to adults: only a small decline in
today's daily sleep (eg USA8 - 1.7% increase in adults sleeping <6h/day
since 1975); only a slow BMI increase limited to those habitually sleeping
<6h 2; little evidence of short-sleep related cardiovascular or other
illnesses unless sleep is <6h 9.

1. Carter PJ,Taylor BJ, Williams SM, Taylor RW. Longitudinal analysis
of sleep in relation to BMI and body fat in children: the FLAME study. BMJ
2011; 242: d2712

2. Horne J. Obesity and short sleep: unlikely bedfellows. Obes Rev 2011;
12: e84-94.

3. Hiscock H, Scalzo K, Canterford L, Wake M. Sleep duration and body mass
index in 0-7 year olds. Arch Dis Child 2011; doi:10.1136/adc.2010.204925

4. Wake M, Price A, Clifford S, Ukoumunne OC, Hiscock H. Does an
intervention that improves infant sleep also improve overweight at age 6?
Follow-up of a randomised trial. Arch Dis Child 2011; 96:526-532.

5. Reilly JJ, Armstrong J, Dorosty AR, Emmett PM, Ness A, Rogers I, Steer
C, Sherriff A. Early life risk factors for obesity in childhood: cohort
study. BMJ 2005; 330: 1357-1363

6. Matricciani L, Olds T, Williams M. A review of evidence for the claim
that children are sleeping less than in the past. Sleep 2011; 34:651-659

7. Terman LM, Hocking A. The sleep of schoolchildren: its distribution
according to age and its relation to physical and mental efficiency. J
Educ Psychol 1913; 4: 138-147.

8. Knutson KL, Van Cauter E, Rathouz PJ, DeLeire T, Lauderdale DS. Trends
in the prevalence of short sleepers in the USA: 1975-2006. Sleep 2010; 33:
37-4

9. Horne J .The end of sleep: 'Sleep debt' versus biological adaptation of
human sleep to waking needs Biol Psychol 2011 87: 1-14